A. Field of the Invention
The present invention relates to energy-efficient photoreceptors, which are constructed with an ultra-low-power transimpedance amplifier apparatus and photodiode in a pulse oximeter application. The energy-efficient photoreceptor apparatus and transimpedance amplifier apparatus disclosed herein provide increased sensitivity, dynamic range, speed, and energy efficiency relative to known photoreceptor devices and allow for a significant reduction of power consumption requirements. In addition, the use of analog processing after the photoreceptor also lowers power consumption.
B. Description of the Related Art
Pulse oximetry is a fast, noninvasive, easy-to-use, and continuous method for monitoring the saturation of components such as oxygen or carbon monoxide in the blood of an organism such as an animal, non-human primate or human patient. Pulse oximeters for blood-oxygen saturation detection operate by comparing transmission characteristics of red and infrared light emitting diodes (LEDs) light through a patient's finger with a photoreceptor. The wavelength and strength of the light that passes through the finger provides information on what proportion of the hemoglobin in the blood is dark red and deoxygenated versus bright red and oxygenated. The modulation of the oximeter signal with arterial diameter due to blood pressure variations in between heartbeats helps separate blood transmission characteristics from the unmodulated tissue background.
A pulse oximeter having a photoreceptor is ubiquitous in modern medicine for non-invasively measuring the percentage of oxygenated hemoglobin in a patient's blood, by comparing transmission characteristics of red and infrared LED light, through their fingers with a photoreceptor. In modern medical practice, a patient's blood-oxygen level is considered one of the important vital signs of the body along with the more traditional ones, such as blood pressure, heart rate, body temperature, and breathing rate. Pulse oximeters provide early information on problems in both the respiratory and circulatory systems. They are widely used in intensive care, operating rooms, emergency care, birth and delivery, neonatal and pediatric care, sleep studies, and in veterinary care.
The most frequent use of pulse oximeters is in the field of anesthesiology. Tissue oxygenation and, consequently, blood saturation are of extreme importance to anesthesiologists because they administer narcotics to the patient to suppress the central nervous system. This administration stops the patient's desire to breathe and places her in a state where she can no longer meet oxygen demands on her own. In addition, anesthesiologists administer muscle relaxants, which stop the ability to breathe and permit airways to collapse. Thus, it is necessary to restore breathing through intubation and artificial respiration. In a sense, the anesthetist becomes the controller for the patient's respiratory system, and the blood-oxygen level provides the best feedback variable.
In an additional to blood-oxygen level detection, detection of carbon monoxide is also increasingly desired. Carbon monoxide is a tasteless, odorless, invisible gas that can build up in enclosed areas where fuels such as natural gas, gasoline, fuel oil, or wood are burned. When an organism inhales carbon monoxide, it begins to replace the oxygen that is normally carried in the blood, which leads to carbon monoxide poisoning. Carbon monoxide poisoning can cause headaches, dizziness, or nausea in humans. If the exposure to carbon monoxide continues, a person may lose consciousness and even die. Carbon monoxide poisoning can be hard to identify. The symptoms can also be caused by several other illnesses. Treatment for carbon monoxide poisoning involves bringing blood oxygen levels back to normal. It is important that an affected person or animal be removed from the area where carbon monoxide may be present and begin oxygen therapy if needed. In the context of carbon monoxide detection, a corresponding apparatus can be used to detect the saturation of carboxyhemoglobin and methemoglobin which are indicative of carbon monoxide poisoning in an organism's blood. Here, the received photocurrent consists of light of multiple wavelengths, rather than only two wavelengths as in oxygen saturation detection. However, like blood-oxygen detection, the light is received from the specimen, transduced from a photocurrent to a stream of electrons, and amplified in order to determine saturation, as in pulse oximetry.
In addition to the applications discussed above, there is a growing demand today for small, low-power, and cheap pulse oximeters and carbon monoxide detectors suitable for many novel and exciting portable, wearable, wireless, and networked medical applications where power consumption needs to be minimal, and real-time detection is important. For instance, home-care monitoring for elderly or chronically ill patients over the Internet is gaining popularity as a continuous and flexible alternative to costly medical supervision in hospitals and nursing homes. Moreover, the military is seeking solutions to remotely monitor the health of soldiers in the battlefield by using light and durable sensor tags attached to their bodies along with radio transceivers to enable wireless monitoring. Other potential applications for such cheap and portable biomedical sensors will also include athlete or farm animal monitoring, emergency patient transport, and wireless sensor networks. Reducing the power consumption of such sensors is a critical step in such applications as power directly dictates battery life, size, and cost which in turn influence the dimensions and price of the overall device. The explosion of wireless networks having a transceiver device such as Bluetooth, 802.11a, 802.11b, 802.11g, Zigbee (802.15.4), and cellular telephones in today's world has increased the appetite for having medical information constantly available via devices wirelessly connected to the internet and or to secure data bases in hospitals.
One known pulse oximeter is disclosed in U.S. Pat. No. 4,773,422 (the '422 patent) to Isaacson, et al. The '422 patent discloses an electronic apparatus for sensing the percentage of constituents in arterial blood and employs a logarithmic amplifier built with bipolar transistors and means for subtracting ambient light signals. However, the '422 patent does not teach or suggest the benefits of low-power consumption and energy efficiency that the present invention provides. These mechanisms include: distributed gain amplification, adaptive loop gain control and unilateralization employed in a transimpedance amplifier apparatus and energy-efficient photoreceptor apparatus, the use of MOS transistors operated in the subthreshold regime to implement a logarithm on standard microelectronic chips, the use of analog processing to lower power consumption, and other benefits described below.